The present invention generally relates to nondestructive inspection methods and systems. More particularly, this invention relates to a method and system for nondestructively determining the fiber volume of a composite material, such as fiber-reinforced polymer matrix structure.
Composite materials generally comprise a fiber reinforcement material embedded in a matrix material, such as a polymer or ceramic material. The reinforcement material serves as the load-bearing constituent of the composite material, while the matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. In view of the important role that fibers play in a composite material, fiber volume fraction is an important factor for determining the load carrying capability of a composite structure, an example of which is a low-density epoxy-graphite retaining ring taught in commonly-assigned U.S. Pat. No. 5,068,564 to Frank. Fiber volume fraction is, by definition, the amount of fibers making up the volume of a composite structure, and is an important measurement made to determine the quality of a composite structure and its ability to safely operate over its design life.
The most common method for determining fiber volume fraction in a polymer matrix composite material is acid digestion, which determines the weight fraction of fiber material in the material by etching away the polymer matrix material to leave only the fiber material. While accurate, this method is destructive since the material being evaluated is dissolved during the process. Therefore, while very useful for measuring the manufacturing process capability, acid digestion cannot be used to measure the fiber content of an actual part that will see service. Therefore, various nondestructive examination (NDE) techniques have been considered. For example, fiber contents of composite structures have been estimated on the basis of measuring elastic constants using ultrasonic wave propagation. These techniques measure the stiffness of the composite structure and, with a related calculation, determine the fiber content of the structure. However, a shortcoming of ultrasonic wave propagation techniques is that they require smooth and flat surfaces for precise ultrasonic measurements, a requirement that is not met by many composite structures. Also, these tests require precise ultrasonic measurements of time and position that are not possible with standard production-grade ultrasonic equipment and scanners.
Another limitation of ultrasonic techniques is that they are not suitable for inspecting composites that have a protective coating. An example of such a composite structure is the aforementioned composite epoxy-graphite retaining ring taught by Frank. A fiber volume fraction for the internal load-bearing section of the retaining ring is needed to verify that the ring will meet design requirements for full life. However, the addition of one or more protective layers affects the wave propagation on which ultrasonic measurements are based. As a result, ultrasonic techniques are generally impractical for use on many production composite structures.
In addition to fiber volume fraction, the presence of internal flaws in composite structures affects the life of these structures. The inspection of large metal components has led to the development of sophisticated x-ray, penetrant, and ultrasonic technologies for detecting surface and volumetric defects. However, composite product designs and manufacturing methods can create different types of defects than those created during the manufacture of metal structures. For example, composite structures can contain excess volumetric porosity that, if undetected, can lead to a drastically shortened component life. Therefore, nondestructive methods have also been developed for measuring the porosity content in composite components during their development for use as a design factor and during the manufacturing process to predict component life.
The most common method for measuring porosity in composite structures is the aforementioned acid digestion technique. When employed to determine porosity, the weight percent of matrix material and fiber material are measured separately by selectively etching only one of the materials. With knowledge of the mass density of the matrix and fiber materials, the percent porosity can be readily calculated. However, as discussed above in reference to measuring fiber volume fraction, acid digestion techniques are destructive and therefore their use is limited to a process control tool where either an entire component or a portion thereof can be sacrificed to measure the manufacturing process capability. It follows that acid digestion techniques cannot be employed to assess actual components that have or will see service.
As with efforts to measure fiber volume fraction, NDE techniques developed to nondestructively estimate porosity content have included sound attenuation. One approach is to calculate the acoustic scattering caused by porosity in a composite structure. For example, the use of attenuation slope measurements has been suggested for estimating porosity. Immersion-based attenuation measurement techniques have been proposed that correct for transducer diffraction and sound transmission losses. Attenuation slope measurements have been determined to be sensitive to the shape or aspect ratio of the pores, leading to the need for different coefficients to estimate the porosity content of composite structures produced by different composite construction techniques. Additional corrections have been identified for attenuation measurements made using focused immersion transducers, such as a correction for surface roughness losses and spatial filtering to correct for frequency-dependent focusing effects.
While the applicability of using ultrasonic attenuation techniques to estimate porosity has been demonstrated in laboratory settings, limitations exist for their practical use in manufacturing processes. For example, existing ultrasonic techniques require precision scanning of two transducers collecting data at multiple frequencies. Depending on the attenuation slope calculation method used, collecting the ultrasonic information needed to analyze porosity can require two or more scans of the composite structure. Because two transducers are required for these measurements with their positioning axes, and since most immersion tanks designed for metal inspection only have one transducer manipulator, the development of immersion tanks with two fully-controllable transducer manipulators would be required to implement existing ultrasonic attenuation techniques for use with composite components. Another shortcoming is the complexity of the calibration, measurements, and calculations required by these techniques.
In view of the above, it would be desirable if nondestructive methods existed for accurately measuring the fiber volume fraction and porosity in composite structures that can be readily implemented in a manufacturing setting.